In the paper web entering the calender, there are variations due to functional inaccuracies in the wet end of a paper machine, i.e., the headbox, the forming section and the press section, and in the drying section. In addition to the primary function of the calender, i.e., the finishing of the surface properties of the web, the calender is also used to correct the above-mentioned inaccuracies in the quality of the web. Thus, the function of the calender must be adjustable, i.e., the profile of the calendering nip must be controllable to compensate for these defects.
Conventionally, the profile adjustment at the calender is conducted by changing the diameter of a metal-faced nipping roll or the nip force locally and/or by using so-called variable-crown rolls, whereby the nip force profile can be adjusted as desired. However, these rolls have a relatively large need for rotational power, due to sliding frictions inside the roll, and furthermore, the rolls are structurally very complicated and thereby also expensive to manufacture.
The change of the diameter is conventionally conducted by heating the mantle of the metal roll locally from the outside or cooling the mantle of the metal roll locally from the inside. Attempts in local heating of the roll mantle include, e.g., hot air blowing, infrared heating and various electric heating elements. Disadvantages of this local heating method include, however, the large size of the heating devices used in relation to the roll, and the need for separate supports and transfer devices. After a web break, cleaning up of the calender is rendered difficult by the small gap between the roll and the heat control device(s), web scraps being easily accumulated in the gap in connection with web breaks.
Another important drawback has been that a change in the diameter of the roll mantle has required relatively large temperature differences, wherein even large variations have been possible in the temperature of the nipping roll in the axial direction. This, in turn, has clearly affected the gloss of the paper, i.e., more glossy and less glossy stripes may have occurred in the paper web.
With respect to related prior art, reference is made to U.S. Pat. No. 4,658,716 which describes a calender roll equipped with several infrared heaters spaced from each other in the axial direction of the roll. The purpose of the infrared heaters is to adjust the diameter of the calender roll to compensate for cross-directional variations in the paper web entering the calender nip. In other words, the profile of the calender nip is adjusted by heating the metal roll from the outside. However, problems associated herewith are generated, as discussed above.
Furthermore, as an example of one method according to the prior art, reference is made to a method described in Finnish Patent Application No. 961816, (corresponding to International Publication No. 97/41298) wherein the local properties of the polymeric surface are influenced by heating and cooling the coating with an external temperature control device.
An example of internal heating of the roll mantle in zones is described in Finnish Patent Publication No. FI 70358, which corresponds to German Patent Application No. DE 3033482 and U.S. Pat. No. 4,425,589.